System reliability considerations in probabilistic risk assesment of nuclear power plants

The frequencies and consequences of severe accidents at nuclear power plants are examined using a systematic procedure called probabilistic risk assessment (PRA). These accidents may be initiated by equipment malfunctions, operator errors or external initiators such as earthquakes, floods and tornadoes. It is in the case of the external events that the structural system reliability concepts are utilized. Taking the seismic risk analysis as an example, this paper discusses the different elements of the analysis — hazard analysis, fragility evaluation, systems analysis and risk quantification — and examines how the structural system reliability methods are applied. Areas requiring further investigation by the PRA analysts are indicated and the data and research needs are identified.